This invention relates to a fuel supply control method for internal combustion engines at the start thereof, and more particularly to a method of this kind which supplies the engine with a required amount of fuel commensurate with the temperature of the engine to thereby enhance the startability of the engine.
In an internal combustion engine equipped with fuel injection valves, fuel injected into an intake pipe by each of the fuel injection valves is carried by intake air flowing in the intake pipe and drawn together with the intake air into a corresponding cylinder via a corresponding intake valve. At the start of the engine, part of the fuel injected into the intake pipe adheres to wall surfaces of the intake pipe in the vicinity of the intake valve, and gradually evaporates with the lapse of time to be supplied into the cylinder with delay in such a manner that part of the fuel adhering to the intake pipe wall surfaces evaporates to be drawn into the cylinder during a suction stroke of the engine in the cycle in which the fuel is injected, and the remaining fuel evaporates to be drawn into the cylinder during a suction stroke in the next cycle or during a suction stroke in the cycle subsequent to the next cycle. The lower the temperature of the intake pipe the higher percent of fuel adheres to the intake pipe wall surfaces and the longer time the injected fuel takes to evaporate. On the other hand, when the engine temperature is raised as the engine is subjected to several times of combustion or when the engine rotational speed increases so that vacuum is developed in the intake pipe, the percentage of fuel adhering to the intake pipe wall surfaces becomes lower.
It has been empirically recognized that the amount of adhesion of fuel to the intake pipe wall surfaces, i.e., the evaporation characteristic of fuel on the intake pipe wall surfaces largely depends upon whether or not the intake pipe temperature is higher than a certain critical value (approximately 9.degree. C.). To be specific, we have conducted experiments to find the following fact: Provided that the required amount of injected fuel per each cylinder at cranking engine rpm of 150 rpm is 100 when the intake pipe temperature is higher than the above critical value (after the engine has been warmed up), the same required amount is 30 when the engine has reached a completely fired state (at 600 rpm) after the engine rotational speed has been increased by initial firing. On the other hand, when the intake pipe temperature is lower than the critical value (i.e., when the engine is in a cold state), the fuel adhering to the intake pipe wall surfaces takes long to evaporate due to the low intake pipe temperature, and accordingly the required amount of injected fuel per each cylinder has to be 50 even when the engine has reached a completely fired state (at 600 rpm), while the same required amount is 100 at the cranking engine rpm of 150.
In view of the above described evaporation characteristic of the injected fuel, it has conventionally been proposed, e.g., by Japanese Provisional Patent Publication (Kokai) No. 57-206736, to determine a value of the fuel injection period for fuel injection valves in dependence on the engine temperature, that conforms to the above described evaporation characteristic of the injected fuel, and correct the determined fuel injection period value by means of a correction coefficient which decreases at a fixed rate with a rise in the engine rotational speed.
According to this proposed method, however, since the correction coefficient decreases at a fixed rate with a rise in the engine rotational speed, it is difficult to smoothly attain complete firing of the engine when the engine is started in a cold state, often resulting in failure of smooth starting of the engine.